Polybenzimidazole dispersed polymer coated nanowires as efficient electrolytes for proton exchange membrane fuel cells

In this study, polymer-coated anisotropic inorganic nanowires dispersed in PBI matrix were introduced to construct 1D proton conducting channels within PBI. Ionic-liquid and solvothermal methods were used for the synthesis of ZrO2 and W18O49 NWs, which were coated with PVPA and PDDA polymers to increase their proton conductivity. Our results showed that, prepared membranes have amorphous nature due to the dominating presence of PBI. SEM analysis revealed the average thickness of membrane of about 36 µm. TG/DTA analysis detected lower weight loss of W18O49 NWs (total 2.8%) compared to ZrO2 NWs (18%). Proton conductivity analysis showed that, PDDA/W18O49 NWs possess relatively 4 times higher proton conductivity (4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \times $$\end{document}×10−4 Scm−1) compared to PDDA/ZrO2 NWs (1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \times $$\end{document}×10−4 Scm−1) at 80 ℃. In addition, PDDA-coated W18O49 NWs dispersed PBI membranes showed the highest fuel cell current density (1.2 A/cm2) and power density (215 mW/cm2) at 150 ℃ after 24 h which is nearly 2.5 times higher than pure PBI membrane. In addition, they exhibited the lowest in-situ proton resistance of about (0.47 Ω) compared with that of pure PBI membrane (0.8 Ω). Our results are introducing new concepts towards the development of thin and efficient polymer electrolyte membranes for PEM fuel cells.

Currently, carbon supported platinum-based electrodes (Pt/C) are widely employed in fuel cells due to their high oxygen reduction rate (ORR) and mass activity.While for the membrane, many kinds of polymers were investigated as the base matrix for PEMs including perfluorosulfonic acid-based membranes (ex: Nafion) 12 .Nafion has good protonic conductivity.However, it loses its conductivity at high temperatures affecting the power generation efficiency.Consequently, a humidifier is needed, implying a separate humidifier unit and control device, which increases the size and cost of the system.Otherwise, membrane structure gradually deteriorates, resulting in a significant increase in resistance.Furthermore, because perfluorinated ionomer (PFI) structurally contains fluorine, its production and disposal has became an environmental issue.Another example is polybenzimidazole (PBI)-based electrolytes, which requires no humidification 13 .In addition, it possesses outstanding properties such as good chemical and thermal stabilities, excellent film-forming properties, superior toughness, promising mechanical characteristics and sufficient gas impermeability.However, pure PBI polymers have poor proton conductivity, making the doping process with strong acid (ex: phosphoric acid (PA)) inevitable.By using PA-doped PBI electrolytes with high molar ratio against PBI unit (Phosphoric acid doping level: PADL about 10-15), it is possible to make non-humidified PEMFCs.However, PA-doping leads to membrane strength deterioration due to the excessive hydrophilicity and PA leaching resulting in Pt dissolution and carbon corrosion.Recently, the effect of 1D and / or 2D anisotropic materials for capturing PA and forming continuous PA channels for improving proton conductivity without increasing PA doping level was reported 7 .
In this study, we attempted to introduce anisotropic inorganic materials with enhanced PA affinity utilizing anionic/cationic polymer coating to maintain the proton conductivity of PA-PBI with reduced PADL of 8, by incorporating a variety of solid proton conductors to form composite PBI membranes 14 .Specifically, two kinds of inorganic nanowires (NWs), ZrO 2 NWs and W 18 O 49 NWs as inorganic fillers coated with poly (vinylphosphonic acid, PVPA) ionic polymer and poly (diallyldimethylammonium chloride, PDDA) cationic polymer were used.Basically, PA acts as an anion in the membrane and can electrically interact with the NWs or the polymer coated on the NWs, forming an efficient 1D proton channel on the surface.It is expected that different inorganic wire/ polymer combinations will affect not only the PA affinity in the film, but also the dispersibility, morphological properties, and even mechanical properties of the membrane.Therefore, we performed a multifaceted evaluation of the fillers prepared in each combination and the PA-PBI electrolyte membranes to which they were added and investigated their effects on the performance of non-humidified PEMFC.

Methods
Preparation of Zirconium oxide nanowires (ZrO 2 NWs) ZrO 2 NWs were prepared using an ionic-liquid route as previously reported 15 .Firstly, (3.36 g) of Zr(OPr n ) 4 was added to (60 mL) EG under magnetic stirring under ambient conditions.Then (4 g) BMImBF 4 was added to the above solution.After that, the formed mixture was transferred into a 100 mL Teflon-lined autoclave which was heated at 160 ℃ for 46 h.After cooling down to room temperature naturally, the formed white precipitate was collected and washed 5 times by EtOH via centrifugation, then dried under vacuum at 60 ℃.Finally, dried powder was heat-treated at 500 ℃ for 2 h.

Preparation of Tungsten oxide nanowires (W 18 O 49 NWs)
W 18 O 49 NWs were prepared by a solvothermal method in literature 16 with slight modifications.(500 mg) of WCl 6 was dissolved in (60 mL) EtOH until clear yellow solution is obtained.Then, the formed solution was inserted into a 100 mL Teflon-lined autoclave heated at 180 ℃ for 24 h.After natural cooling, the formed blue powder was collected, washed 4 times by EtOH by centrifugation, and dried at 60 ℃ in a vacuum oven.

Surface modification of the prepared NWs
Proton conductivity of the prepared NWs was improved through PVPA and PDDA surface coating as the following: NWs of both types were dispersed in (20 mL) distilled water (D.W.) using sonication for 1 min.Then, PVPA and PDDA polymers were added to the NWs' dispersion (polymer ratios were 20 wt.%, 50 wt.%,and 65 wt.% compared with the NWs).After that, the mixtures were left under continues stirring overnight.Finally, the coated NWs were collected, washed 1 time by D.W. and dried under vacuum at 60 ℃.

Preparation of polymer-coated NWs' dispersed PBI membranes
Many polymer-coated NWs' dispersed PBI membranes (polymer-coated NWs = 2 wt.%) were prepared via a simple casting method developed by Maegawa et al., as shown in Fig. S.1 ref 17,18 .First, (9.62 mg) of NWs was dissolved in (18.81 g) DMAc using ultrasonication for 1 h.Then, (4.71 g) of PBI was added to the above solution and left under magnetic stirring for another 1 h.After that, sonication took place for 30 min to form a clear and homogonous solution.Then, final solution was casted in petri dishes (diameter = 97 mm) and heat-treated for 24 h using a programmed gradual heat increase from 60 to 120 ℃.Finally, obtained membranes were treated with hot water for 5 h at 90 ℃ to remove any residual DMAc.As a reference, pristine PBI membrane (with no NWs) was prepared by the same method for comparison.4 membranes are presented in this study named, W 18 O 49 /PBI, PVPA/W 18 O 49 /PBI, PDDA/W 18 O 49 /PBI, and Pure PBI membrane.The thickness of the prepared membranes was in the range of 35 to 55 µm.

Phosphoric acid (PA) doping of membranes
Before fuel cell measurements, membranes (2 cm × 2 cm) were doped with PA doping level of 8 mol, (PADL = 8) by immersion in PA, then membranes were stretched between two glass slides at 60 ℃ for 1 h, PADL was calculated using Eq.(1): where W o and W doped are the weight of membranes before and after PA doping.While M wt represents the molecular weight.

PA leaching analysis
To analyze PA retention by the membranes, PA leaching test was carried out according to our previous study 19 .Membranes with PADL = 8 were exposed to steam (from ion exchange water at 100 ℃) and after specific time intervals, the remaining amount of PA (after gentle wiping) was calculated by measuring the weight loss over time.

Swelling ratio & water uptake test
The manufactured membranes were placed into 85% PA solution.Subsequently, swelling ratio and water uptake percentage were assessed according to Eqs. (2, 3), respectively.Prior to the immersion in PA, the initial weight (m o ), thickness (T o ), and area (A o ) of the membranes were recorded.Following PA doping at 60 ℃, the weight (m doped ), thickness (T doped ), and area (A doped ) were determined following a careful wipe (to remove excess PA solution).The PA-doped membranes were then dried in an oven at 100 ℃ for 1 h, and the mass (m dry ) was measured.

Membrane-electrode assembly (MEA) and evaluation of fuel cell performance
1 × 1 cm 2 and 1.5 × 1.5 cm 2 Pt/C electrocatalyst sheets were utilized as the anode and cathode, respectively.To evaluate fuel cell performance, PA-doped membranes (PADL = 8) were sandwiched between the two electrodes to fabricate MEAs.Then fuel cell testing system (Auto PEM system, Toyo Technica, Japan) supplying H 2 and O 2 at a flow rate of 100 mL min −1 to the anode and cathode, respectively was employed, experiments were performed at 150 ℃ (anhydrous conditions).

Characterization of the prepared samples
Phase and crystallinity were examined by X-ray diffraction (XRD) analysis performed with a (Rigaku, Ultima IV X-ray diffractometer, Japan, utilizing Cu-Kα radiation (λ = 1.54 Å) and operating at 30 mA -40 kV.Scanning electron microscopy (SEM) conducted with a (HITACHI S-4800, Japan), accompanied by an energy-dispersive X-ray spectroscopy (EDX) unit (Oxford Instruments, AZtecLive Lite, Ultim Max 40), was employed to determine the morphology, average length, and diameter of NWs.The surface charge of NWs (Zeta potential) at various pH levels was investigated using an (ELS-Z1NT analyzer, Photo OTSUKA ELECTRONICS, Japan).To assess the mechanical strength of the membranes, a Tensilon apparatus (RTF-1250, Japan) was used, with a tensile test speed of 1 mm min −1 .X-ray photoelectron spectroscopy (XPS) was carried out on a (PHI Quantera SXM Scanning X-ray Microprobe, ULVAC-Phi, Inc., Japan), for analyzing elemental composition and chemical states.Thermal stabilities of the NWs were assessed in air through TG/DTA analysis conducted on (Rigaku, Thermo plus EVO2, Japan) with heating rate of 10 ℃ min −1 .Lastly, temperature-dependent proton conductivity measurements were carried out via AC impedance spectroscopy over a frequency range of 1 Hz-10 MHz using (Solartron, SI 1260) under dry N 2 conditions.

Characterization of the prepared NWs and membranes
XRD pattern of the prepared membranes is presented in Fig. 1.As clearly observed, for pure PBI membrane, a broad peak is detected at 2θ = 25 °, which is reflecting its amorphous nature.While for W 18 O 49 NWs' dispersed membranes, a slight shift towards the smaller angle is recorded, which is attributed to the change in the internal structure and / or disruption of the ordered stacking of PBI 20,21 .114), (020), (315), and (523).The recorded peaks are in good agreement with those recorded for the monoclinic phase of Tungsten oxide (JCPDS No. 712450) 22 .It is worth mentioned that unassigned peaks are related to other metal oxide forms of Tungsten  To estimate the net surface charge of the NWs affecting the affinity with coating polymers which in turns affect PA interaction and homogeneity in the membranes, Zeta potential/surface charge analysis of bare ZrO 2 NWs, W 18 O 49 NWs, and after their PVPA coating is carried out and results are summarized in Table 1.In the acidic medium (actual fuel cell working environment), W 18 O 49 NWs possessed net negative charge (Zeta potential = − 26.96).On the contrary, ZrO 2 NWs had net positive charge (Zeta potential = + 24.35).Thus, selecting the proper polymer for NWs' coating is important to facilitate the electrical PA affinity and creating continuous 1D H + conducting channels on NWs, as well as to fabricate homogeneously dispersed membranes.Taking into account that, PVPA possesses anionic charge while PDDA exhibits cationic charge, the suitable polymer/NWs combinations can be expected as follows: ZrO 2 /PVPA and W 18 O 49 /PDDA.PVPA (having protonic acid groups) is expected to have high proton conductivity itself.On the other hand, PDDA has the potential to electrically interact with anionic PA when it is incorporated in the PA-doped membranes resulting in an effective proton conductive channel, although PDDA itself has negligible proton conductivity.
Figure 4 shows the XPS analysis of the prepared PDDA/W 18 O 49 /PBI membrane.Figure 4a shows the survey study, where many peaks associated with all substantial elements are observed along with their atomic percentages.While the deconvoluted peaks of the main elements (W, O, N, and C) are shown in Fig. 4b-e.For W 4f., two main peaks at (34.4 and 37.3 eV) are detected, corresponding to (4f.7/2 and 4f.5/2 ), respectively, confirming the presence of W +6 state 25 .While for N 1s, the two peaks recorded at (397.6 and 399 eV) are due to the W-N bond configuration.For O1s, the characteristic peak of oxygen in metal oxides was observed at (531.9 eV) and confirming the presence of hydroxyl groups.Finally, for C1s, one main peak at (284 eV) was detected corresponding to C = C bond 26 .

Proton conductivity of the prepared polymer-coated NWs
To confirm the effect of polymer coating of NWs on proton conductivity, a set of proton conductivity measurements at a relative humidity of 80% were carried out.Figure 5a  www.nature.com/scientificreports/

Fuel cell performance analysis
Figure 6a-d presents the current-voltage curves of the prepared membranes, while power density curves are shown in Fig. 7a-d.PDDA/W 18 O 49 / PBI membrane showed the maximum current density of 1.2 A/cm 2 Fig. 6d and its open circuit voltage (OCV) (at current density = 0 A/cm 2 ) was maintained for 24 h.In addition, it showed the maximum power density of 215 mW/cm 2 after 24 h which is 2.5 times higher than pure PBI membrane Fig.7d.This result can be attributed to an improved protonic conductivity.It is worth to mention that OCV was slightly different for the prepared membranes, with a maximum value of about (0.85 V after 8 h) exhibited by W 18 O 49 / PBI membrane Fig.6b, while a minimum value of (0.65 V) was recorded for pure PBI membrane Fig.6a.The decreasing of OCV over time is attributed to the gas leakage between the anode and cathode electrodes due to membrane penetration 27 .Thus, the polymer coated NWs may bring morphological and mechanical improvements, resulting in a reduced gas permeability.Another observation is related to the slope of the linear part of the  I-V curve associated with membrane resistance, PDDA/W 18 O 49 / PBI membrane exhibited the lowest resistance (slope) value of about (0.47 Ω) followed with W 18 O 49 / PBI membrane (0.63 Ω), this smaller slope compared with that of pure PBI membrane (0.8 Ω), further confirming the improved protonic conductivity with respect to pure PBI membrane due to the absence of polymer-coated NWs.The dominant reason for this improvement is the PA coordination on the PDDA/W 18 O 49 surface derived by the positively charged PDDA, forming 1D proton conducting channels on an anisotropic wire surface.www.nature.com/scientificreports/ The OCV dramatically decreased after 24 h in case of pure PBI and PVPA/W 18 O 49 / PBI membrane Fig.6c.This significant decrease in OCV is due to either deterioration of catalyst performance over time due to corrosion or infiltration caused by PA leaching which results in an inadequate reaction at the three-phase interface 28 or H 2 /O 2 gas permeation due to membrane rupture and / or cracks.Thus, it can be concluded that, NWs' dispersion in PBI membranes led to both PA retention as confirmed by Fig. 8  The constant current stability performance of PDDA/W 18 O / PBI and pure PBI membranes with a constant current density of 0.2 Acm −2 for 24 h is depicted in Fig. 9.The recorded potential of PDDA/W 18 O 49 / PBI membrane tend to increase over time, this behavior can be related to PA leaching from PBI membrane and PA infiltrating into PBI electrode interface as seen in Fig. 8, up to 3 h, PA retention of PDDA/W 18 O 49 / PBI membrane was better than that of pure PBI membrane, however with time, PA tend to leach from the membrane causing activation of the catalyst and increasing of the protonic conductivity and the associated electrochemical reactions at the 3-phase boundary leading to elevated potential value over time.This behavior is typical in PEM fuel cells 29,30 .Figure 10a,b shows the calculated electrode and electrolyte resistance from electrochemical impedance spectroscopy analysis of the prepared membranes.It is obvious that, at 24 h, PDDA/W 18 O 49 / PBI membrane possessed the lowest electrode and electrolyte (membrane) resistance which is favorable for high current and power densities.It is worth to mention that, values of electrolyte resistance in Fig. 10b are smaller than those shown in Fig. 6, that's because in Fig. 6, values are corresponding to both electrolyte and interface resistances, while in Fig. 10b are solely due to the electrolyte resistance 31 .

Conclusion
ZrO 2 and W 18 O 49 NWs were firstly prepared by ionic-liquid and solvothermal methods and then coated with PVPA and PDDA polymers to reduce the required amount of PA doping in the PBI membrane through increasing their protonic conductivity.Then, the prepared nanowires (2 wt.%) were dispersed in PBI matrix to form 4 polymer electrolyte membranes for PEM fuel cells.
Our results showed the importance of surface charge in fabricating membranes with uniform dispersion of nanowires.In addition, mechanical strength analysis revealed that, nanowires' dispersion in PBI matrix increased the tensile strength of the formed membrane with respect to pure PBI membranes.PDDA/W 18 O 49 / PBI membrane showed the maximum current and powder densities of 1.2 A/cm 2 and 215 mW/cm 2 after 24 h which is 2.5 times higher than pure PBI membrane.The improved fuel cell performance of PDDA/W 18 O 49 /PBI membrane than that of PVPA/W 18 O 49 /PBI, is attributed to the construction of proton conductive pathway because of the increased PA affinity, and the suppression of membrane mechanical degradation (leading to membrane cracks, gas crossover, and lower OCV).The advantages of the introduction of anisotropic nanowires in membrane thinning, mechanical properties improvement, surface charge role in membrane uniformity, and type of coating polymer for high PA affinity were revealed.

Figure 2 .
Figure 2. SEM analysis before heat treatment of (a) bare ZrO 2 NWs (b) bare W 18 O 49 NWs, (c) Cross-sectional analysis of W 18 O 49 / PBI membrane, and (d) its EDX mapping for N 2 element.

Figure 9 .Figure 10 .
Figure 9. Potential over time at a constant current density of 0.2 Acm −2 for 24.

Table 1 .
Zeta potential and net surface charge of the prepared ZrO 2 NWs and W 18 O 49 NWs at different pH.